This invention relates to a system for defrosting a condenser used in freeze drying devices, and more particularly, to an electric defrost means insertable into the structure of a condenser for the removal of frost and ice accumulated on its outer surface during the operation of an associated freeze dryer.
The well-known process of freeze drying has provided an efficient technique for the dehydration of a wide variety of products, producing an end product virtually identical to the original material minus its water content. Briefly, four conditions must be obtained to accomplish proper freeze drying; the product to be dehydrated must be solidly frozen, a heat source must be employed to provide the heat of sublimation necessary to drive the water content of the material directly from its solid state to the vapor state, a condensing surface is required and, finally, the system must be provided with a vacuum.
The present invention involves the condenser portion of a freeze dryer, which provides the surface on which the water content of the material released by sublimation is condensed in the form of frost or ice. Once the material to be dried has been completely dehydrated, the condenser is covered with a layer of ice or frost which must be removed before another freeze drying run can be conducted. The present invention provides a unique means of defrosting the condenser, which solves several of the problems associated with prior art attempts to accomplish this result.
In the past, defrosting the condenser has been accomplished by a variety of means, including placing electric heaters on the outside of the condenser, blowing hot air over the ice on the surface of the condenser, reversing the refrigeration cycle in the material flowing through coils around the condenser, or simply allowing the ambient heat in the air to melt the ice away.
Defrosting of commercial refrigeration systems, including refrigerators and freezers, has been accomplished by techniques such as disclosed in U.S. Pat. No. 2,755,371 (Jackson). In Jackson, heating units are inserted into tubes disposed within selected bends in the coils of the refrigeration system, to remove accumulations of ice on the coils. This system of defrosting is not acceptable for use with condensers in freeze drying systems, however, since the efficient transfer of thermal energy between the heating means and the outer surface of the coils is accomplished only if the coils are completely filled with refrigerant fluid. As discussed below, it is undesirable, both in terms of cost and efficiency, to flood the condenser with refrigerant fluid during either the freeze drying or defrosting process.
The primary consideration in the defrosting devices or methods mentioned above is to accomplish the removal of ice and frost from the condenser as quickly as possible. In the past, it was thought that rusting and corrosion of the condenser could be avoided if the condensate was completely removed and the condenser surface cleaned and dried directly after defrosting. However, it has been found that corrosion and rusting begin shortly after the defrost cycle begins, even though a physical examination of the condenser shows only the collected ice to be present on the surfaces. The problem occurs under the surface, where a fluid interface exists between the condenser and the ice layers, which actively rusts the condenser until the outer ice layers break up and fall away. This is particularly a problem where corrosive materials, having a relatively high acidic or alkaline content, are dried. It is readily apparent that if the fluid interface between an ice layer and the condenser consisted primarily of a corrosive acid or base, the surface of the condenser would deteriorate quickly unless the outer ice layers were quickly broken away. Many of the prior methods of defrosting condensers mentioned above do not remove the ice or frost quickly enough to significantly reduce such rusting and corrosion of the condenser.
Another problem associated with prior art defrosters, particularly the hot air blowers and the electric heaters placed on the outside of the condenser, is that such devices tend to raise the temperature of surrounding portions of the freeze dryer adjacent to the condenser, especially the manifold in which the condenser is housed. As mentioned above, a requisite of freeze drying is the provision of a controlled heat input to provide the appropriate heat of sublimation to the frozen material undergoing drying. By raising the temperature of the elements of the dryer near the condenser and the receptacles containing frozen material to be dried, the turn-around time required before subsequent freeze drying runs may be made is lengthened by the time it takes such heated areas of the dryer to cool down to ambient temperatures. In addition, blowers and electric heaters are much more expensive to purchase and operate than the defrosting means of the present invention, while little or no increase in efficiency of operation is provided over the present invention, as discussed below.